Grinding of materials in grinders having rolls with stepwise-increasing speeds



March 12, 1968 A. L. VERDIER 3,372,878 GRINDING OF'MATERIALS IN GRINDERSHAVING ROLLS WITH STEPWISE-INCREASING SPEEDS 4 Sheets-$h eet 1 FiledSept. 15. 1964 ITH A. L. VERDIER GRINDING OF MATERIALS IN GRINDERSHAVING ROLLS W STEPWISE-INCREASING SPEEDS March 12, 1968 4 Sheets-Sheet2 Filed Sept. 15. 1964 Fig. 3

MKuw MW w M, M4, Maw

March 1-2,- 1968 GRINDING Filed Sept. l5, 1964 A. L. VERDIER OFMATERIALS IN GRINDERS HAVING ROLLS STEPWISE-INCREASING SPEEDS WITH 4Sheets-Sheet 5 A. L. VERDIER March 12,1968 4 I GRINDING OF MATERIALS INGRINDERS HAVING ROLLS W STEPWISE-INCREASING SPEEDS ITH 4 Sheets-Sheet 4Filed Sept. l5, 1964 u as,

J T5 39F L 5:0 N9 02 g \F v 82 I x J I 5 2K QQ 2 I mm mm United StatesPatent Oiifice 3,372,878 Patented Mar. 12, 1968 3,372,878 GRINDING OFMATERIALS IN GRINDERS HAV- ING RQLLS WITH STEPWISEJNCREASING SIEEDSAlldl Louis Verdier, 6 Ave. Daniel-Lesueur, Paris, deine, France FiledSept. 15, 1964, Ser. No. 396,6? Claims priority, application France,Sept. 2%, 1963, 948,254 12 Claims. (Cl. Mfr-16) The invention relates tothe grinding or milling of materials in paste form, such as have to betreated in particular in the printing ink, paint, chocolate, pigment,soap, rubber and plastics materials industries.

This grinding process consists not only in reducing the dimensions ofthe solid particles present in the paste but, according tocircumstances, also in, for instance, dispersing them or wetting them,kneading or homogenising the paste or grinding and dispersing the clotsor lumps that are formed. One endeavours to obtain at the end of theoperation as regular a distribution as possible or pulverulent solidmaterials in materials in liquid phase.

To this end, grinders or mills having rolls with stepwise-increasingspeeds are currently used in the various industries. These machinesinclude a first roll (or slow roll) and at least two other rolls, eachof these being pressed against the preceding roll and rotating morequickly than the latter and in the opposite direction. A compositionconsisting of solid and liquid materials pre viously mixed together isintroduced into the hopper of the machine, which is arranged above thefirst two rolls. The pasty composition enters between the latter,adheres for the greater part to the second roll by reason of its greaterspeed, passes between the latter and the third roll, to which it mainlyadheres, and is thus conveyed, passing in succession between the rollspressed one against the other, to the outlet of the machine, where it iscollected from the last roll by a pick-up knife.

Such grinders generally comprise three rolls, but some machines, inparticular those which are used in the soap and chocolate industries,have more.

In some industries there is an increasing dislike for grinderscomprising rolls having speeds which increase stepwise from roll toroll, because it is impossible at present to obtain a constant andregular output of a product having the desired properties. It is foundin particular that the output depends on the preliminary operation ofmixing the composition which is fed into the hopper. In one and the samegrinder, without altering either the speed of the rolls or the pressurewith which they are applied one against the other, totally differentoutputs are obtained by changing from one composition to another. It haseven been observed that successive operations on one and the samecomposition do not necessarily give the same output or the same grindingquality. Moreover, the prod not leaving the machine is not homogeneous.

It is an object of the invention to remedy these drawbacks by employingnew adjusting means acting on certain phenomena which are inherent inmachines having rolls with progressively increasing speeds. Theinvention makes it possible for reproducible grinding qualities to beobtained with mixtures of solids and liquids, that is to say, it enablesa product of constant predetermined quality to be obtained in amanufacturing process.

According to the present invention a batch of paste, which may bepreviously mixed or not, is introduced into a machine, and is made topass in closed circuit between the rolls thereof while adjusting thetemperature, the speed and the pressure of the rolls one against theother, for a time suflicient for there to be obtained in the hopper atthe end of the operation, a paste having the desired quality ofdispersion, which paste is then removed by a final passage between therolls.

This method of grinding can be carried into eifect by means of animproved grinder which also forms part of the invention and whichcomprises, in combination, means for regulating the wall temperature ofthe rolls and means for varying the speed of the first two rolls incontinuous fashion. According to a further feature of the invention, tothis improved grinder there are preferably added judiciously arrangedscrapers, a suitable system for adjusting or regulating the speed of therolls and the pressure thereof one against the other, and/ or a de vicefor mixing the paste in the hopper.

The temperature of the rolls, the pressure thereof one against the otherand their speed must be adjusted in order to obtain a given temperaturewithin the paste, so as to maintain the viscosity of the paste at asuitable value. In some cases, this temperature must be maintained belowa fixed limit, either so as to avoid altering the taste or the colour ofthe paste-in particular in the case of the manufacture of chocolate orpaints respectively or so as to avoid the transformation by heat ofcertain constituents of the paste, such as of the natural or syntheticresins which are used in the manufacture of certain printing inks, inthe case of which it is necessary to avoid starting polymerisation.

By permitting pastes to be worked so as to obtain a very fine and veryhomogeneous product without excessive heating, the method of grindingaccording to the invention makes it possible to eliminate the operationof conching in the chocolate industry, this operation consisting incontinuously stirring the paste which leaves the grinder for severaltens of hours. This method, which thus make it possible to obtaindirectly a paste containing particles of sugar which are disintegratedto the desired fineness, regularly coated with cocoa butter andintimately mixed with the other constituents, also forms part of theinvention, as also do the machines which enable the said method to becarried into effect.

The invention also comprises the application of the new method ofgrinding to the operation of phase substitution which is known in thepigment industry as the flushing process, this application consisting inintroducing a mixture of water and precipitated pigments into the hoppertogether with a liquid substitution phase and in working the mixtureobtained in this way in the manner already described. Thus, the samequality of dispersion which hitherto has been obtained only after aseries of complex operations is now obtained directly.

The following description with reference to the accompanying drawings,which is given by way of non-limitative example only, will enable thevarious features of the invention and the manner of carrying them intoeffect to be clearly understood, any arrangement appearing either fromthe text or from the drawings coming within the scope of the saidinvention.

In the drawings:

FIGURE 1 shows diagrammatically in cross-section, a grinderincorporating the improvements provided by the invention;

FIGURE 2 is a View similar to FIGURE 1, but showing another embodiment;

FIGURE 3 is a diagrammatic elevational view of a grinder;

FIGURE 4 is a cross-sectional view on the line IV-IV of FIGURE 3,showing means for adjusting the tightening forces applied to the rollsand for controlling the distances between them;

FIGURE is a diagram of an alternative constructional form of anadjusting and controlling system for the rolls.

The grinder shown in FIGURE 1 comprises three rolls 1, 2, 3 rotating inthe direction of the arrows 1b, 2b, 312 at speeds which increasestepwise from roll to roll, the first two rolls 1, 2 being surmounted bya hopper 4 and the high-speed roll 3 being equipped with a pick-up orcollecting knife 5.

In the known grinders, the rolls are hollow and cooled by a flow ofwater passing through their shafts 1a, 2a, 3a which are hollow. In thegrinder shown, the hollow shaft of each roll is connected to aclosed-circuit flow of fluid at controlled temperature and this enablesthe temperature of the walls of each roll to be regulated independentlywhile avoiding incrustation. The hopper 4 has double walls as indicatedat 4a, 4b and between the double walls there flows in closed circuit afluid at controlled temperature which enters through a connection 40 andissues through a connection 4d. It is thus possible to regulate thetemperature of the wall of each of the rolls and of the hopperindependently.

A thermometric sensing device 6 is also provided and this enables thetemperature of the paste contained in the hopper to be constantlychecked. This thermometric device is advantageously supplemented orreplaced by a device (not shown) enabling the viscosity of the paste tobe continuously checked. Such devices are known per se, there beingvarious types of viscosimeters which can be obtained on the market.

A scraper 7 is fast with two sealing devices 8 which prevent any drop inpressure, at the opposite ends of the line of contact between the rolls2 and 3, in the paste passing between the latter. Such devices aredescribed in my copending patent application Ser. No. 376,467, PatentNo. 3,332,630. The assembly can be moved about an axis 9 so that thescraper 7 can be applied either against the roll 3 in order to throw thelayer of paste adhering thereto back on to the roll 2 or, on the otherhand, against the roll 2, in order to throw the layer of paste adheringthereto back on to the roll 3.

With these arrangements it is possible, by applying the scraper againstthe roll 3, to cause the paste to pass in closed circuit between therolls while independently adjusting the wall temperature of the latterand of the hopper 4, as a function of the temperature or the viscosityof the paste contained in the hopper. The paste which is introducedbetween the rolls 1 and 2 adheres for the greater part to the roll 2, itpasses between the latter and the roll 3, adheres again for the greaterpart to the latter and, when the scraper 7 is located in the positionshown, it is thrown back by the latter on to the roll 2 which returns itto the hopper, the paste passing below the wall 4a, 4b of the hopper,which wall is for this purpose maintained slightly spaced from the roll2. In order to remove the paste at the end of the operation, the scraper7 is made to bear on the roll 2 so as to deliver the residual layer ofpaste still adhering to the roll 2 onto the roll 3; the collecting knife5 detaches the paste adhering to the roll 3 in known manner.

So that the paste which passes in a closed circuit between the rolls mayundergo a uniform treatment throughout, scrapers 10 and 11 are providedwhich are applied against the rolls 1 and 2, respectively, so as todetach the layers of paste which adhere thereto and to return them tothe mass or main body of the paste. Moving scrapers are also providedand these constantly scrape the walls of the hopper in order to detachtherefrom the peripheral layers of paste which would have a tendency tocling thereto. In the drawing, the longitudinal walls of the hopper(that is to say the walls parallel to the axes of the rolls) are shownas having their cross-section in the form of circular arcs whose centreis located at the spindle 13. The scrapers 12 are made to bear on theselongitudinal walls and driven with a reciprocating movement about thesaid spindle 13, by means of arms 12a. Other scrapers (not shown) may beprovided, bearing on the end walls of the hopper and set in motion byany suitable means in order to scrape the said end walls.

A rotary mixing device 14 is also provided comprising two worm sectionswith opposite threads which start from the respective ends of the lengthof the rolls and end at the middle of the hopper, the said sectionsrevolving on a shaft 15. The threads of the two screws are oriented in adirection such that the rotation thereof constantly shifts the pastelocated in the vicinity of the ends of the rolls towards the middle ofthe length of the latter. The worms can be mounted to be freelyrotatable on their shaft 15 and the movement thereof is then produced bythe ascending current 16 which occurs in the paste above the line ofcontact of the rolls 1 and 2. The shaft 15 is advantageously mounted atits opposite ends on the end walls of the hopper by means of retractablecups which permit rapid placing in position and removal thereof.

An ascending current such as indicated at 16 occurs in all machinesequipped with rolls, whether the rolls have stepwise-increasing speedsor not, and is manifested by a constant rotary movement of the paste inthe hopper. This current has not been utilized heretofore. The applicanthas found that it is strong enough to drive the mixing device 14-.

The mixing device 14 may also be driven in rotation by a mechanicaldevice (not shown). The arms 12a of the scrapers 12 are thenadvantageously driven with a reciprocating movement by the mixing device14 through the medium of a suitable transmission. In the drawing thistransmission is represented diagrammatically by a connecting rod 13awhich converts the rotary movement of the device 14 into an oscillatingmovement of the arms 12a about their spindle 13.

The rolls of known grinders are generally driven by gears and the ratioof their speeds is constant. In FIG- URE 1, the rolls 1 and 2 arecontrolled separately by two continuous speed-changing devices showndiagrammatically at 17 and 18. It is thus possible to control the speedsof the first two rolls independently. The roll 3 is driven by the motorthrough a train of gears (not shown). This ear train could also bereplaced by a continuous speedchanging device.

The arrangement for pressing the rolls one against the other ortightening them, which is not shown in its entirety in FIGURE 1,comprises means enabling the distance between the rolls to be measuredand the forces tightening the rolls or pressing them one against theother to be adjusted in order to obtain a desired distance between them.Such means are described in my co-pending patent application Ser. No.352,158 and will be described again with reference to FIGURES 3 to 5.They comprise very sensitive distance or spacing gauges showndiagrammatically at 19 and 29, which are interposed between the bearingssupporting the shafts of the rolls; the tightening forces and the speedsof the rolls are adjusted so that the distances or spacings as read onthe gauges assume the desired values. This adjustment is effected bymeans of a regulating system (not shown in FIGURE 1, but which will bedescribed with reference to FIGURE 4) enabling the distances to beadjusted to a predetermined optimum value, account being taken of theavailable power of the machine.

I shall describe now with reference to FIGURES 3 to 5 the adjusting andcontrolling means disclosed in my above-mentioned co-pending patentapplication Ser. No. 352,158.

The grinder shown in FIGURES 3 and 4 is similar to the grinder shown inFIGURE 1, at least in parts related to the rolls thereof. Three rolls191, 102, 103 are rotated in the directions indicated by the arrows, atgraduated speeds. Portions of paste which are forced between the rollsor are adhering thereto are shown at 104, 104a and 1ti4b. A scraper forremoving the paste from the third roll is shown at 105.

The roll 131 is journalled in two fixed side plates 1ti1a and 1411b. Theroll 192 is journalled in two side plates 102a and 102!) which arepivotally mounted on a spindle 1%. The roll 3 is also journalled in twoside plates 1h3a and 1031b which are pivotally mounted on a spindle 107.An offsetting device, shown diagrammatically at 110, enables the spindle6 to be displaced in height.

The rolls are driven rotatively at different speeds by a drivingarrangement represented diagrammatically in FIGURE 4 by a train of gears111.

The tightening system comprises two single-acting jacks 1&8 and 109, thecylinders 108a, 109a of which are connected to the fixed side plates10112 and 101a respectively, and the piston rods 108b, 10912 of whichare connected respectively to the movable side plates and 163a of thecollecting roll. A hydraulic fluid drawn from a reservoir 112 iscompressed at a constant pressure by a pump 113 and delivered by a pipe114 into a pressure regulating valve 115 which supplies in parallel, onthe one hand, a pipe 116 equipped with a pressure gauge 116a and leadingto the chamber 108d of the jack 108 and, on the other hand, a pipe 117equipped with a pressure gauge 117a and supplying the chamber 1090. ofthe jack 109 through the medium of a pressure reducing valve 118.

Each of the valves 115 and 118 enables the pressure to be regulateddownstream thereof by delivering a part of the fluid into a dischargepipe 115a and 118a which returns it to the tank 112.

In the operation of this tightening system, when the rolls are at astandstill the tightening or closing action thereof is controlled byregulating, by means of the valve 115, the pressure supplied by thepipes 116 and 117 to the chambers 198d and 10%., until the rolls 1111,102, and 103 are pressed one against the other in the position shown inthe drawing. The pressures read on the pressure gauges 116a and 117a arethe same. When the rolls rotate, entraining the paste 1114, the pressurewhich they exert on the latter is stronger at the non-driven end, thatis to say the end where the plates 101a, 102a and 103a are located, thanat the end driven by the gears 111. The pressure in the chamber 109dmust therefore be reduced by means of the valve 118 so as to decreasethe tightening action at the non-driven end.

In order to obtain regulation of the tightening action which is preciseand which can be automatic, the arrangement shown in FIGURE 4 will beused. A differential pressure gauge 119 is connected by pipes 120 and121 to the chambers 1118c and 10% which are filled with oil, so that thepressure gauge 119 indicates the difference in the pressures prevailingin these chambers 1080 and 109C.

Between the ipes 120 and 121 there is arranged a by-pass 122 which canbe closed by two cocks 120a and 121:: and which leads to a variableoutlet 123 controlled by a cock 1230. An auxiliary pressure gauge 124 isconnected by a cock 124a to the pipe 120.

In order to use the system, the machine being at a standstill and therolls moved apart, the two chambers 1118c and 10% are first filled withoil by means of cocks such as 125 (FIG. 3) and these cocks are closed.When the preliminary tightening is carried out (rolls at a standstill)in the usual manner, the two cocks 120a and 121a are opened and theoutlet 123 is controlled by means of the cock 123a in such manner thatthe pressure in the chambers 163a and 1090 does not reach a value liableto damage the differential pressure gauge 119. The pressure in therespective chambers 1418c and 109s is substantially the same since thepistons of the two jacks advance simultaneously and at the same speed,bringing the rolls nearer to one another, and it is read on the pressuregauge 124. When the three rolls are in contact and are tightened orpressed together with the appropriate pressure, which is read on thepressure gauges 116a and 117a, the cock 123a is first closed so that thepressures in the chambers 108a and 11190 are strictly equalized owing tothe by-pass 121, and then the cocks 120a and 121:: are closed. In thisposition, the rolls are applied one against the other and the oilpressure is precisely the same in the chambers 1080 and 109C. Thepressure gauge 119 therefore indicates a zero differential pressure.

When the rolls are set in rotation, the paste 104 first passes betweenthe rolls 1411 and 1132 and then between the rolls 192 and 1%, movingthem apart to a greater extent at the end where the side plates 101b,10211, 10% are located than at the end where the side plates 101a, 102a,103a are located. The result is that the pistons of the two jacks shiftslightly towards the bottom of FIG- URE- 4, reducing the pressure in thechambers 108C and 11190, the pressure decreasing more in the chamber108s than in the chamber 10%. The differential pressure gauge 119therefore indicates a lower pressure in the chamber 1118c than in thechamber 10%. By reducing the pressure in the chamber with! of the jack109 by means of the valve 118, the tightening force applied by this jackto the rolls will be reduced and, consequently, the pressure wh ch thelatter exert on the paste at the non-driven end will also be reduced;the reaction of the paste therefore slightly increases the distancebetween the rolls at this end and the piston of the jack 199 movesslightly towards the bottom of FIGURE 3, thus reducing the pressure inthe chamber 1:190. When the differential pressure between the chambersItiitic and 1096 has reached a zero value, which is indicated by thedifferential pressure gauge 119, the correction of the regulation by thevalve 118 is stopped and, in this way, it is ensured that the sum of thedistances between the rolls 1191 and 102 and between the rolls 1% and1113 is the same on the side where the jack 169 is located and on theside where the jack 108 is located. In one stroke equality has beenachieved between the pressures exerted by the rolls on the paste at bothends.

Instead of operating the valve 118 manually in order to bring thereadings of the differential pressure gauge 119 to zero, use willpreferably be made of the differential pressure between the chambers1ti3c and 1090 in order to actuate a control device operating the valve11% in such manner as to maintain this differential pressure at a Zerovalue. For example, the ditferential pressure gauge 119 may be providedwith electrical contacts controlling a servo-motor which actuates thevalve 118, but it is also possible to use other types of control usingthe differential pressure as an imput signal. This control of the valve118 in response to the differential pressure as indicateddiagrammatically in FIGURE 4 by the chain-dotted line 126.

It will be understood that moving apart of the rolls under the action ofthe paste passing between them has the effect of shifting the pistons ofthe two jacks 10S and 109 towards the bottom of FIGURE 4.. Thesemovements which, in the embodiment described, are possible because thevalves 115 and 118 maintain fixed pressures in the chambers 108d and109d, are very small. The differences in the movement of the two pistonsare of the order of one micron in printing ink grinders. However, theditferences in pressure which result therefrom in the chambers N and101C are sufficient to be indicated by the differential pressure gauge119 or to produce a control signal for the valve 113. Any variation inthe reaction of the gears 111, due to a change in the speed of the rollsfor example, causes a variation of the differential pressure at thepressure gauge 119, enabling equal compression to be re-establishedthroughout the length of the rolls by means of the valve 118.

However, in this connection, it should be noted that a change in thespeed of the rolls causing a variation in the ditferential pressure willbe desired and called for by, and will therefore be dependent on, thewill of the person conducting the operation or the programming thereof.On the other hand, a difference of homogeneity in the paste or adifference in temperature, manifesting itself as a difference in theviscosity of the mixture, cause unexpected variations in thedifferential pressure. One of the advantages of the invention is that itpermits permanent correction of the differential pressure as a functionof the variations of the reactions of the control devices; whichthemselves depend on the variations of the power consumed during thesame operation.

In the alternative constructional form shown in FIC- URE 5, the elementsperforming the same function as in FIGURE 4 are designated by the samereference numerals. The by-pass 122, which in FIGURE 4 is intended topermit the reduction of the volume of the chambers 108a and 1090 and toestablish equality of the pressures in the chambers upon the rolls beingtightened when at a standstill, is improved in this case. The cocks 120aand 121a are replaced by electric valves 127 and 128 and the outlet 123is replaced by a pressure accumulator 129 of the hydro-pneumatic type;moreover, the by-pass 122 is connected to a cock 130 enabling the volumeof oil in the whole of the installation to be adjusted on starting upand, consequently, enabling the cocks 125 of FIGURE 3 to be dispensedwith.

On the preliminary tightening operation in the rest state, the twoelectric valves 127 and 128 are opened and the oil expelled by thereduction in volume of the chambers little and 1090 is delivered to theaccumulator 129. The preliminary tightening operation having beencompleted, the electric valves 127 and 128, which are controlled by asingle electric circuit (not shown), are closed simultaneously and it isthus ensured that there will be equal pressures in the two chambers 1080and 1090.

Also shown in FIGURE are the motor 113a of the pump 113 and a filter113!) located at the intake of this pump, in the reservoir 112.. Thevalve 118 is an electric valve controlled by the differential manometer119 through the line 126. The valve 115 is also an electric valve. Amanometric contactor 131 is provided which is connected to the pipe 116and which, by means of electrical connections (not shown) controllingthe valve 115 or the pump 113, enables the placing of the installationunder pressure to be interrupted in the event of the pressure increasingabnormally.

It will be observed that in FIGURE 4 the pressure gauge 124 measures thepressure in the chamber 108c and consequently gives an indication of thespacing adopted by the rolls under the action of the paste interposedbetween them. By operating the valve 115 in such manner as to maintainthis pressure at a given value, it is therefore possible to give therolls the desired spacing. In the constructional form shown in FIGURE 5,regulation of the tightening action as a function of the distancebetween, or spacing of, the rolls is provided, this being representeddiagrammatically by a pressure gauge 132 with electrical contacts, whichis connected to the pipe 120 and, by a line 133, to the regulation valve115 and to a control 133a controlling the speed of rotation of therolls.

Such regulation enables the compression actually undergone by the pastebetween the rolls to be maintained at the desired value. In machinesequipped with rolls and which do not comprise this regulating system,the thickness of the layer formed on the exit or delivery roll such as103 can be determined by controlling the weight of material suppliedduring a given time. It is sufficient to know the specific gravity ofthe material, such as the paste 4-, and to take account of thetangential speed and of the width of the layer recovered form the exitroll. This layer thickness, however, does not give any indication of thevalue of the distance between the last two rolls or between thepreceding rolls. The applicant has found that in a machine having rollswith graduated speeds these distances are distinctly smaller than thethickness of the layer recovered.

The applicant has also found that the reduction of the distances has aconsiderable influence-for a mixture of a given viscosity-on the onehand on the velocity of How within the paste, which is considerablyhigher than in contact with the rolls and on the other hand on themaximum pressure within the paste, which increases very rapidly when thedistance between the rolls decreases.

Thus, the variations towards an increase or a decrease in the distancehave a considerable influence on the qualities of the products obtained.

In known grinders it is therefore possible to control the quality of theproduct obtained and, by successive experiments, to determine theconditions to be observed for the preparation of the mixture, itsviscosity and its other characteristics, as well as for the variousadjustments of the machine, so as to obtain a product of the desiredquality, but no means are available for knowing and producing the mostfavourable distances between rolls. Although it is possible to reproducea given manufacturing operation exactlyprovided that all the factors arestrictly identical to those of the reference manufacturing operationthere may be considerable variations in the distance between the rollsif one of the factors concerning the engagement or introduction of thematerial is diiferent. It will be understood that under these conditionsit is impossible--if the composition of the mixture or the nature of themanufacturing operation is changed to obtain distance values identicalto those which have been given the best results during previousexperiments or manufacturing operations.

The regulation represented diagrammatically in FIG- URE 5 by thepressure gauge 132 and the line 133 enables the distance between therolls to be maintained automatically at the desired value by acting, onthe one hand, on the tightening pressure by means of the valve and, onthe other hand, on the speed of rotation of the rolls by means of thecontrol 13311 as a function of this distance, which is measured by thepressure in the chamber 108C.

The pressure gauge 132, for example, comprises a plurality of electricalcontacts arranged to transmit to the line 133 signals which are used bythe valve 115 to supply to the chamber 108d a tightening pressure whichwill maintain the chamber 108c at the desired pressure, the pressure inthe chamber 10% being maintained equal to the pressure in the chamber1080 by the control device described above. If the valve 115 cannotrespond to a call for an increase in the tightening pressure (forinstance because the latter is already equal to the pressure of the pump113), the signal transmitted over the line 133 actuates the control13311 for slowing down the rolls. The effect of this slowing down is toreduce the quantity of paste which is introduced between the rolls, theresult is that the distance between the rolls decreases until thepressure in the chamber 1080, which measures this distance, has reachedthe desired value. The contacts of the pressure gauge 132 are adjustableand this enables the pressure which is automatically maintained in thechamber 1980 by the regulating arrangement to be adjusted at will.

It will be apparent from the above description with reference to FIGURES3 to 5, that the chambers 1080 and 109c-together with pressure gauges124 or 132, and 119-form very sensitive distance or spacing gaugesbetween the rolls. Such gauges make it possible to control the spacingof the rolls by increasing and decreasing the tightening and speeds ofthe rolls. Tightening of the rolls can be varied by actuating valves 115and 113. The speeds of the rolls can be varied in a manner Well knownper se, by actuating control means of the driving arrangement showndiagrammatically at 111.

The operator of the grinder machine can check a char- 9 acteristic ofthe pastenamely the viscosity or temperature thereof-on checking device6 while the paste is worked in closed circuit by the machine, and adjustthe wall temperatures of the rolls and control the tightening and speedsof the rolls to adjust spacings thereof, as a function of the checkedcharacteristic i.e. in the sense that will produce desired evolutionthereof.

In my above-mentioned co-pending patent application Ser. No. 352,158 andin the above description with reference to FIGURES 3 to 5, anarrangement is provided such that when all the tightening action of themachine has been used and the spacing must nevertheless be furtherreduced, the speed of rotation of the motor which drives all the rollssimultaneously is reduced, so as to decrease the volume of paste whichis introduced between the rolls, the speed of rotation being reduced bymeans of a regulating system which receives signals which are a functionof the distances between the rolls and which acts on the one hand on thetightening forces and, on the other hand, on the said speed of rotation.

When, in accordance with FIGURE 1, the rolls 1 and 2 are controlledseparately by the speed-changing devices 17 and 18, it is possible, inorder to effect reduction of the distance between the rolls when thetightening effect cannot be further increased, to reduce only the speedof the first roll, the other rolls being then slowed down in the eventof this reduction in speed not sufficing to obtain the desired distanceor gap.

It will be understood that in FIGURE 1 the spacing between rolls 1 and 2can be controlled independently of the spacing between rolls 2 and 3,since the rotation speeds of the rolls can be controlled independently.Each of spacing gauges 19 and 20 can comprise pressure chambers 108a and1090 and pressure gauges like pressure gauges 124 or 132 and 119 ofFIGURES 4 and 5.

For example, it is possible to use a regulating system similar to thatof FIGURE 5 but acting selectively on the speed-changing device ordevices 17 and 18 and on the motor (not shown) which simultaneouslydrives these speed-changing devices and the roll 3. In response to asignal calling for reduction of the distance or gap, the regulatingsystem first increases the tightening effect and then, if the demandpersists beyond the possibility of tightening, gradually reduces thespeed of rotation, be ginning with the slowest roll. In response to ademand for an increase of the distance or gap, the regulating systemgradually accelerates the rolls, beginning with the fastest roll, eachof the rolls being accelerated to the speed suitable for the operationin progress and then, if necessary, reduces the tightening effect untilthe said demand is met. However, the power available for driving therolls must not be exceeded.

It is therefore necessary to provide for the introduction into theregulating system, of parameters which are a measure of the differencebetween the power consumed by the rotation of the rolls or of each ofthem and the power available for this rotation, and/or the correspondingdifferences in torque. Devices emitting signals as a function of theseparameters are known to those skilled in the art.

These signals will be fed to the regulating system as prohibitionsignals. On receipt of such a signal, instead of the regulating systemeffecting the normal operation which it should have effected if thissignal had not been received, it will effect a following operationreducing the power consumed until the said normal operation has becomepossible. The regulating system will perform a sequence of operations(action on the tightening effect and action on the speed of the rolls)in response to an input signal (demand for a reduction or increase inthe distance or gap between the rolls) it being possible for the orderof these operations to be changed by means of prohibition signals (riskof exceeding the power). Those skilled in the art will know how toconstruct such regulating systems.

If the input signal constitutes a demand for the reduction of distancewhen the first roll, for example, is already consuming all its availabledriving power, or almost all of its driving power, the regulating systemwill not perform the normal operation of increasing the tighteningeffect, but will slow the first roll down until the power consumed hasdecreased sufficiently to cancel the prohibition signal and to permitthe tightening effect to increase. If the regulating systemsimultaneously receives a demand for an increase in distance and aprohibition signal, it will not perform the normal operation ofaccelerating the rolls, but will first reduce the tightening effect inorder to decrease the power consumed to the point at which theprohibition signal will be cancelled and will only then perform thenormal acceleration.

It will be seen that the arrangement described enables the optimumdistance to be maintained between the rolls, while causing the rolls toturn at the maximum speed permitted both by the power available for thetightening of the rolls and by the power available for the rotation ofeach of them.

So as to understand clearly the usefulness and the advantages of thearrangements described, it is necessary to examine in greater detail thephenomena which occur during the working of paste, on the one hand ingrinders of the types heretofore known and on the other hand in grindersaccording to the invention.

In the methods of grinding hitherto employed, it has been foundsatisfactory to introduce into the hopper of the grinder a compositionconstituted by solid and liquid components, which were previously mixed,and then to start the machine so that the composition is entrained andpasses between the rolls in succession, the product being collected bydetaching it from the last roll by means of a pick-up knife. The solemeans of adjustment provided on such a machine were its speed ofrotation and the tightening force applied to the rolls; there was noawareness of the fact that a gap is produced by the passage of the pastebetween the various pairs of rolls.

The mixture introduced into the hopper, considered on the scale of thefinest particles, is a heterogeneous mass comprising zones of differentdegrees of denseness (in accordance with the proportions of theconstituents present in each zone) and of irregular temperature andviscosity. The paste introduces itself between the first and secondrolls, the distance between which is unknown, then between the secondand the third, the distance between which is also unknown (and so onbetween the succeeding rolls if the machine comprises more than threerolls). No means are available for adapting the distance between therolls to the characteristics of the portion of the paste which passesbetween them, although in order to obtain uniform grinding of the pasteit would be necessary not only to adapt the distance between the firsttwo rolls to the characteristics of the portion of paste passing betweenthem, but also to adapt the distance between the following rolls to thecharacteristics of the paste when these have been modified by theworking to which the paste has been subjected by preceding rolls.

The viscosity of the paste in any given zone depends on the ratiobetween the volume of liquid and the total surface area of the solidparticles present in that zone. The sizes of the solid particles, ineach zone of the paste, are distributed over a granulometric range ofgreater or lesser extent according to the nature of the product, thegrinder used and the fineness sought. If the tightening effect on therolls is increased in order to reduce the dimensions of the largestparticles, the finest particles will he reduced at the same time and,consequently, their total area will be increased and the viscosity willbe thereby increased. This phenomenon of increase of the viscosity withthe fineness of the grinding is demonstrated by, for example, themanufacture of chocolate, in which a paste (cocoa, sugar and cocoabutter) is introduced into the hopper of a five-roll grinder and apulverulent product is recovered at the outlet of the machine.

It certainly seems, therefore, that the viscosity, like the granulometryof the solid particles in dispersion, the proportions of the variousconstituents and the other characteristics which result therefrom evolvein the various zones of the paste on each passage between two rolls.Among the characteristics which evolve in this way on each passage, themost important appear to be the viscosity and the adherence orstickiness of the paste, which mainly affect the mechanical actioninvolved in the engagement of the paste between two rolls. It isprobably the uncontrollable variation of these characteristics, betweentwo different batches fed into the machine and in the various zones ofone and the same batch, that causes the disparities in manufacture whichhave been found hitherto.

The temperature of the paste has a considerable influence on itsviscosity and the friction of the rolls causes considerable heating,which is counteracted in known machines by circulating cold water. Thisenables excessive heating to be avoided but since it is subject to, inparticular, incrustation and irregularities in the supply pressure, itdoes not permit of controlling the temperature of the rolls. Theresultant variable temperature of the rolls, which acts in successiononly on one face of the layer of paste entrained by each roll, can causeexcessive local heating without the whole volume of paste being affectedby this heating. The pastes will therefore be heated in varying degreesand will have higher or lower heterogeneous viscosities and this againhas an effect on the distance between the rolls and on the quality ofthe product obtained, every time that a new operation of grinding amixture of the same formula is carried out. Submitting the paste toseveral passages through the machine therefore only accentuates thedifferences which may exist between the zones which have been workedmost and those which have been worked least in one and the same mixture,which differences are not apparent to users of the machine since theyhave at their disposal for the purpose of monitoring the manufacturingprocess, only means which make it possible to examine whether thelargest agglomerates or the largest grains do not exceed a chosen value.Thus, the upper limit of the range of grain sizes of the solids to befound in the composition of the mixtures is fixed in this way, but thereis complete ignorance of what is the lower limit and the distribution ofthe dimensions within this range.

The grinder described with reference to FIGURE 1, on the other hand,makes it possible to effect the simultaneous grinding and dispersion ofsolid particles in liquids in successive batches of constant weight forevery type of mixture, while affording the possibility of reproducingthe cycle of the various phenomena which are necessary for obtaining, atthe end of the operation, a constant quality of the mixture worked withthe latter always remaining in the pasty state.

In fact, the arrangements described make it possible to substitutecontrolled phenomena producing a desired result for the uncontrollablephenomena which occur in known machines. The paste, pas-sing in a closedcircuit between the rolls and constantly mixed in the hopper, can bemaintained at each instant at the viscosity which is most favourable tothe stage of the operation in progress, due to the control of the saidviscosity (or of the temperature of the paste) and to the simultaneousregulation or adjustment of the temperature of the rolls and the hopper,the tightening effects and the speeds of the rolls (making it possibleto influence the heating which the paste undergoes during its passagebetween the rolls, by reason of the considerable pressures andaccelerations brought into play). The concomitant adjustment of thedistance between the rolls enables this to be adapted to the viscosityand to the stickiness of the paste (that is to say to its aptitude forintroducing 12 itself between the rolls) and to the desired particlefinemess.

The following example will enable the possibilities of the grinderdescribed to be appreciated. The example concerns the grinding of amixture which presents the risk of separation of the liquid phase,either because the ratio of the volume of the liquid to the totalsurface area of the solid particles is too large, or because the solidmaterial is poorly wetted.

The composition, which may be previously mixed or not, is introducedinto the hopper 4 and the flow of fluid inside the rolls 1, 2 and 3 isadjusted so as to provide a relatively low temperature, so as not toreduce the viscosity of the paste which is formed and to ensure adequatesurface tension between the solid and liquid components of the paste,and also between the liquid component and the surface of the rolls.

The speed changing devices 17 and 18 are adjusted so as to give therolls 1 and 2 almost equal speeds, the roll 2, however, rotating alittle more quickly so as to ensure the transfer of the layer to theroll 3. The speeds of the rolls 1 and 2 are as high as possible, havingregard to the power of the machine, and the distance between the rollsis adjusted to the greatest value possible, without introducing any riskof the liquid dropping from the rolls (so as to ensure the engagementtherewith of both the largest agglomerates and the liquid phase).

As stated above and fully explained in my co-pending patent applicationSer. No. 352,158, the distance along which the rolls are forced apart bythe material passing between them is a function of the rotation speed ofthe rolls and the hydraulic tightening pressure which is applied to therolls. Thus, the tightening pressure is adjusted to the lowest valuepossible without the liquid material flowing freely through the nipbetween the first and second rolls.

The working undergone by the paste in this first stage modifies certainphysical characteristics of the constituents and the result is a gradualmodification of the mechanical action involved in the engagement of thepaste between the rolls with a consequent effect on the power consumed.This results in increase or reduction in the speed of the rolls, but atthis stage the speeds of the rolls 1 and 2 must remain very close.

In this first stage the solid particles are crushed and broken up; theresult is an increase in the ratio of their total surface area to thevolume of the liquid and the composition begins to form a paste having asignificant viscosity. In the case of particles which are poorly wetted,these are engaged with the liquid between the rolls 1 and 2 by reason ofthe fact that their speeds are almost equal and, as a result, there is agrinding action and homogenization, which also form a paste having asignificant viscosity. In both cases, there comes a point when theviscosity becomes measurable and when it can be checked by means of theviscosimeter located in the hopper 4.

The temperature of the rolls and of the hopper is then increased so asto reduce this viscosity and to enable the distances between the rollsto be reduced gradually. The roll 2 is accelerated to the highest speedpermitted by the power of its speed changing device 18; the hydraulictightening pressure on the rolls 1, 2 is increased and the speed of theroll 1 is reduced until the required distance is obtained between therolls 1 and 2, which distance is the result of the lowering of theviscosity of the paste combined with the increase in the tighteningeffect and the increase in the difference in speed between the rolls 1and 2. The roll 3 is accelerated and its tightening pressure isincreased.

At this stage of the operation (there being a small distance between therolls and a high speed of rotation of the rolls 2 and 3), the phenomenonof heating starts in the very midst of the paste by its passage betweenthe rolls, causing a supply of calories much greater than that which canbe supplied by heat exchange with the Walls 13 of the rolls. It willtherefore be understood that the heating of the rolls and of the hopperby means of a heating fluid which has originally served to reduce theviscosity serves at the same time to start this phenomenon.

It follows that the temperature of the paste, and therefore thedevelopment of its viscosity, is the combined result of heating by thewalls of the rolls and heating by the passage of the paste between therolls (that is to say by consumption of the power developed by themachine). The layers of paste adhering to the rolls are constantlydetached by the scrappers 7, It) and 11 and the paste is constantlymixed in the hopper by the mixing device 14 and detached from the wallsof the hopper by the scrapers such as 12, so that the temperatureindicated by the device 6 or the viscosity indicated by the viscosimeterare actually average characteristics of the paste.

A time comes when this average viscosity or temperature reaches a chosenlimit. The temperature of the fluid circulating in the rolls is thengradually reduced and the supply of calories caused by the passage ofthe paste between the rolls is decreased by increasing the distancesbetween the rolls and/ or reducing their speed, so as not to exceed thesaid limit.

It is obvious that these adjustments must be adapted to each type ofmanufacturing process, in accordance with the programme of operationswhich it is possible to perfect for each type of process, takingadvantage of the arrangements described. The duration of each stage ofthe manufacturing process can be fixed and a control of the times can bearranged.

For example, a program of production can be settled by working a firstbatch of the materials and recording all particulars of the productionprocess, such as the evolution and timing of the wall temperature andspacing of the rolls and the resulting evolution of the viscosity ortemperature of the paste.

Then, the program can be used for working further batches of thematerials successively, that is to say the recorded data can be used tocontrol the wall temperatures and spacing of the rolls, whereby toobtain a number of batches of product having like qualities.

It should be noted that as soon as it has been possible to commence theheating of the paste the temperature of the hopper will have beenbrought as rapidly as possible to a maximum value corresponding eitherto a theoretical viscosity of the mixture, or to a limiting temperatureimposed by the nature of the product. This temperature will bemaintained until the end of the operation or until the moment when themanufacturing cycle requires the use of a different temperature; in thiscase, the fluid supplying the circulation in the hopper has its valuebrought immediately to the new temperature desired.

The grinder of FIGURE 2 comprises the same arrangements as that ofFEGURE l and similar elements are designated by the same references.However, the hopper 21 is larger and surmounts the three rolls 1, 2 and3. The knife 7 is replaced by a knife 22 which can adopt either of twopositions as indicated diagrammatically by the arrows 22a; for the finalremoval of the paste the knife is brought to bear on the roll 2 so as todetach the paste adhering thereto and to deliver it to the roll 3;during the passage of the paste in closed circuit, the knife 22 israised and the paste adhering to the roll 3 is detached by another knife23 bearing on the latter. For the final removal, this knife 23 is raisedto the position 2311, leaving between itself and the roll 3 a gap justsufficient to permit the passage of the paste adhering to the roll; thispaste is detached by the pick-up or collecting knife 5.

This arangement is advantageous for pasty products such as inks, paints,soap, plastics materials and generally speaking in all those industrieswhere it is sought both to Wet, disperse and grind solid particles in aliquid phase, since it enables the volume of the hopper to besubstantially increased without creating dead zones. It also enables theevaporation of solvents to be reduced.

In particular, a machine constructed in accordance with the arrangementof FIGURE 2, which comprises rolls of large diameter and great length sothat it can be charged with a considerable tonnage, can be used forcarrying out the grinding and conching of chocolate.

What I claim is:

1. A process for the preparation of pigments by the method of liquidsubstitution known as the flushing process using a grinder having acharging hopper adapted to receive material and a series of rollsrotating 'at speeds increasing stepwise from roll to roll, pressed oneagainst the other and between which the material passes, comprising thesteps of: V

(a) introducing into the hopper a composition consisting of a mixture ofwater and precipitated pi gments together with the substitution liquid,

(b) causing the composition to pass between the rolls and allowing it toreturn to the hopper in 'a closed circuit While adjusting thetemperature, the speed and the pressure of the rolls one against theother in concomitance, for a time sufiicient for the composition tobecome a paste constituted by a dispersion of the pigments in thesubstitution liquid and having the desired quality of dispersion, and

(c) removing the paste by causing it to pass between the rolls for thelast time and detaching it from the last roll of the series.

2. In a process of wet grinding a batch of material by causing thematerial to pass through a series of nips defined between consecutiverotating rolls while an urging force is applied to the rolls to pressthe same against each other for mutual cooperation along adjacentgeneratriccs thereof, and adjusting the urging force and the rotationspeeds and temperatures of the rolls, the improvement comprising ('a)returning the material to the batch to make a mixture therewith andagain between the nips and back to the batch in closed circuit to form apaste progressively, while checking a characteristic of the paste in thebatch, selected from the group consisting of the temperature and theviscosity of the paste, and adjusting the urging force and the rotationspeeds and temperature of the rolls as a function of the saidcharacteristic, and (b) taking off the paste by causing the paste topass through the nips once again.

3. A process according to claim 2 for the preparation of pigments by themethod of liquid substitution known as the flushing process, wherein thebatch comprises a composition consisting of water, precipitatedpigments, and a substitution liquid.

4. In a grinder having a spread of at least three consecutive rolls withparallel axes, means for applying an urging force to the rolls to pressthe same against each other for mutual cooperation along adjacentgeneratrices thereof, driving means for positively rotating consecutiverolls in opposite directions at speeds which increase from a first rollto a last roll of the spread, a hopper adapted to contain pasty materialfor feeding the material between the first roll and the second roll ofthe spread whereby said material is driven and compressed between theconsecutive rolls successively, means for adjusting the walltemperatures of the rolls, means for adjusting the urging force, andmeans for adjusting the speed of the driving means, the improvementcomprising means in the hopper for checking a characteristic of thepaste, selected from the group consisting of the temperature andviscosity of the paste, means for separately varying and adjusting thespeeds of rotation of the first and second rolls, means adapted toreturn the pasty material from the last roll to the hopper, a helicalrotary mixer for returning the paste at the ends of the hopper to themiddle thereof, and means adapted to take off the pasty material fromthe last roll.

5. A grinder comprising a spread of at least three consecutive rollswith parallel axes; means for applying 'an urging force to the rolls topress the same against each other for mutual cooperation along adjacentgeneratrices thereof; driving means for positively rotating consecutiverolls in opposite directions at speeds which increase from a first rollto a last roll of the spread; a hopper extending from the first to thelast roll, adapted to contain pasty material for feeding the materialbetween the first roll and the second roll of the spread whereby saidmaterial is driven and compressed between the consecutive rollssuccessively; means for adjusting the wall ternperatures of the rolls;means for adjusting the urging force; means for adjusting the speed ofthe driving means; means in the hopper for checking a characteristic ofthe paste, selected from the group consisting of the temperature andviscosity of the paste; means for separately varying and adjusting thespeeds of rotation of the first and second rolls; a scraper above thelast roll, which is applied against the surface of the last roll inorder to return the layer of paste adhering thereto to the hopper; meansfor disengaging said scraper from the last roll and for providing in thewall of the hopper a slit of a height just sufi'icient to allow thelayer of paste adhering to the said last roll to pass; and means adaptedto take oif the pasty material from the last roll outside of the hopper.

'6. A grinder for manufacturing chocolate without conching, comprising'a spread of three consecutive rolls with parallel axes; means forapplying an urging force to the rolls to press the same against eachother for mutual cooperation along adjacent generatrices thereof;driving means for positively rotating consecutive rolls in oppositedirections at speeds which increase from a first roll to a third roll ofthe spread, the three rolls having upper portions which form a bottomfor a hopper adapted to contain pasty material for feeding the materialbetween the first roll and the second roll of the spread whereby saidmaterial is driven and compressed between the consecutive rollssuccessively; means for independently regulating the temperatures of theWalls of the three rolls and the temperature of the inner wall of thehopper; means for adjusting the urging force; means for adjusting thespeed of the driving means; means for separately varying and adjustingthe speeds of rotation of the first and second rolls; a scraper in thehopper and means for shiftin the scraper between a position in which itis applied against the third roll and a position in which it is spacedfrom the third roll by a distance just sufiicient to allow a layer ofmaterial in paste form adhering to the third roll to leave the hopper; apick-up knife applied against the third roll outside the hopper; meansfor detaching the portion of paste adhering to the walls of the hopperand delivering it into the batch of paste; means for mixing the batch ofpaste in the hopper while returning the portion of paste located in thevicinity of the ends of the rolls to the center of the hopper; and meansin the hopper for checking a characteristic of the paste, selected fromthe group consisting of the temperature and viscosity of the paste.

7. In a process of wet grinding a batch of a composition of solid andliquid materials which tend to segregate, by causing the materials topass through a series of nips defined between consecutive rotating rollswhile an urging force is applied to the rolls to press the same againsteach other for mutual cooperation along adjacent generatrices thereof,and adjusting the urging force and the rotation speeds and temperaturesof the rolls, the improvement comprising (a) returning the material tothe batch to make a mixture therewith and again between the nips andback to the batch in closed circuit to form a paste progressively, withthe speeds of rotation of a first on and a second one of the rolls whichdefine between them the first nip of the series that receives thecomposition being adjusted to almost equal values, the urging forcebeing adjusted to the lowest value possible without the liquid materialsflowing freely through the first nip, and the temperatures of the rollsbeing adjusted to a value low enough for the materials to form a pastehaving substantial surface tensions between solid and liquid componentsof the paste and between the liquid components and the surfaces of therolls, until the paste has a significant viscosity, (b) checking acharacteristic of the paste in the batch, selected from the groupconsisting of the temperature and the viscosity of the paste, andadjusting the urging force and the rotation speeds and temperatures ofthe rolls as a function of the said characteristic, and (c) taking offthe paste by causing the paste to pass through the nips once again.

8. A grinder according to claim 4, wherein the said mixer is disposedabove the facing generatrices of the first two rolls, as the result ofwhich the mixer is driven rotatively by an ascending current of pasteproduced in the hopper above said generatrices.

A grinder according to claim 4, comprising a device for setting themixer in motion and knives adapted to scrape the walls of the hopper.

it A grinder according to claim 12, comprising means for delivering tothe regulating system prohibition signals which are a function of thedifference between the power consumed by the rotation of the rolls andthe available power of the grinder, said regulatin system being adaptedto change in response to said prohibition signals, the order of theoperations performed by said regulating system.

11. In a grinder having a spread of at least three consecutive rollswith parallel axes, means for applying an urging force to the rolls topress the same against each other for mutual cooperation along adjacentgeneratrices thereof, driving means for positively rotating consecutiverollers in opposite directions at speeds which increase from a firstroll to a last roll of the spread, a hopper adapted to contain pastymaterial for feeding the material between the first roll and the secondroll of the spread whereby said material is driven and compressedbetween the consecutive rolls successively, means for adjusting the walltemperatures of the rolls, means for adjusting the urging force, andmeans for adjusting the speed of the driving means, the improvementcomprising means in the hopper for checking a characteristic of thepaste, selected from the group consistin of the temperature andviscosity of the paste, means for separately carying and adjusting thespeeds of rotation of the first and second rolls, means adapted toreturn the pasty material from the last roll to the hopper, meansadapted to take off the pasty material from the last roll, andclosed-circuit fluid circulating means for independently regulating thetemperatures of the walls of each roll and the temperature of the wallsof the hopper.

12. In a grinder having a spread of at least three consecutive rollswith parallel axes, means for applying an urging force to the rolls topress the same against each other for mutual cooperation along adjacentgeneratrices thereof, driving means for positively rotating consecutiverollers in opposite directions at speeds which increase from a firstroll to a last roll of the spread, a hopper adapted to contain pastymaterial for feeding the material between the first roll and the secondroll of the spread whereby said material is driven and compressedbetween the consecutive rolls successively, means for adjusting the walltemperatures of the rolls, means for adjusting the urging force, andmeans for adjusting the speed of the driving means, the improvementcomprising means in the hopper for checking a characteristic of thepaste, selected from the group consistin of the temperature andviscosity of the paste, means for separately varying and adjusting thespeeds of rotation of the first and second rolls, means adapted toreturn the pasty material from the last roll to the hopper, meansadapted to take off the pasty material from the last roll, and aregulating system receiving input signals which are a function of thedistance between the rolls and acting, on the one hand, on thetightening References Cited UNITED STATES PATENTS 801,259 10/1905Neurnann 241-159 X 1,697,441 1/1929 Walker 241-159 X 2,134,897 11/1938Theiner M 241-98 Hill 241-67 X Irwin 241-37 X Atkinson 241-37 X Sakataet a1. 241-37 Verdieer 241-159 FOREIGN PATENTS Great Britain.

ANDREW R, JUHASZ, Primary Examiner.

1. A PROCESS FOR THE PREPARATION OF PIGMENTS BY THE METHOD OF LIQUID SUBSTITUTION KNOWN AS THE "FLUSHING PROCESS" USING A GRINDER HAVING A CHARGING HOPPER ADAPTED TO RECEIVE MATERIAL AND A SERIES OF ROLLS ROTATING AT SPEEDS INCREASING STEPWISE FROM ROLL TO ROLL, PRESSED ONE AGAINST THE OTHER AND BETWEEN WHICH THE MATERIAL PASSES, COMPRISING THE STEPS OF: (A) INTRODUCING INTO THE HOPPER A COMPOSITION CONSISTING OF A MIXTURE OF WATER AND PRECIPITATED PIGMENTS TOGETHER WITH THE SUBSTITUTION LIQUID, (B) CAUSING THE COMPOSITION TO PASS BETWEEN THE ROLLS AND ALLOWING IT TO RETURN TO THE HOPPER IN A CLOSED CIRCUIT WHILE ADJUSTING THE TEMPERATURE, THE SPEED AND THE PRESSURE OF THE ROLLS ONE AGAINST THE OTHER IN CONCOMITANCE, FOR A TIME SUFFICIENT FOR THE COMPOSITION TO BECOME A PASTE CONSTITUTED BY A DISPERSION OF THE PIGMENTS IN THE SUBSTITUTION LIQUID AND HAVING THE DESIRED QUALITY OF DISPERSION, AND (C) REMOVING THE PASTE BY CAUSING IT TO PASS BETWEEN THE ROLLS FOR THE LAST TIME AND DETACHING IT FROM THE LAST ROLL OF THE SERIES. 